The use of a carbon-containing material suitable for lithium insertion to make the anode of lithium rechargeable electrochemical cells is becoming widespread, to the detriment of lithium metal anodes. The cycling ability of the cell is considerably improved, as is safety in use. Nevertheless, such carbon-containing materials lead to large losses of cell capacity during the initial cycles. Part of the initial capacity of the cathode is irreversibly lost in forming a passivation layer on the carbon in the anode due to reduction of the electrolyte. This loss increases with increasing size of the material to be passivated. Nevertheless, the instability at low potential of the electrolytes used makes the presence of this passivation layer essential.
Early work carried out in this field related to carbon-containing coke type materials that were crystallized to a greater or lesser extent. Nowadays, most work relates to electrodes based on natural or artificial graphites such as the lithium insertion material. Coke that is partially in the form of graphite or mixed with graphite has been mentioned on several occasions, as have spherical carbons and modified carbons. The physico-chemical modifications described relate generally to surface properties, to the addition of atoms other than carbon in the core of the material, or to the extent to which the carbon is graphitized by heat treatment. Such carbons have not made it possible significantly to reduce the losses of capacity due to the formation of the passivation layer.
Proposals have also been made to compensate for said losses by including a reserve of lithium at the anode end. The lithium may be in the form of a metal sheet stuck to the anode as in patent WO-90/13924, or it may be in the form of lithium inserted within the anode chemically or electrochemically. The first solution has the drawback of mechanically loosening the electrochemical couple because of the lithium that is consumed and that becomes inserted in the carbon by short circuiting, thereby leaving a void. The second solution requires an additional step in the manufacture of the anode. That step is difficult to implement, in particular because of the hazards associated with some of the substances used, and subsequently the resulting anode must be handled using the same precautions as are required for handling lithium metal (no water, . . . ).
Finally, an additional capacity for compensating such losses can be inserted into the cathode in the form of lithium ions by means of an additional step during manufacture of the electrode, which step is expensive and difficult to implement. The resulting product is unstable in the presence of air and of water, and is therefore difficult to handle.